In some areas of the printing industry, there is a need and, perhaps, a requirement that printed images have great detail. Such images are often printed with one or more very fine lines using intaglio printing plates. For example, the currency of most countries as well as many securities and other important documents are printed using intaglio printing plates.
In the past, intaglio printing was typically performed using intaglio printing plates which were flat. The intaglio printing plates consisted of lines or dots recessed below the surface of the plates. The intaglio printing plates were usually engraved by hand and, consequently, were very time consuming to engrave. Also, it was very difficult and time consuming to generate identical plates for multi-color printing processes since the intaglio printing plates were engraved manually.
Attempts have been made in the past to automate the manufacture of the intaglio printing plates by engraving the intaglio images on cylinders. For example, attempts have been made to engrave intaglio images on the cylinders using rotogravure engraving equipment. While such rotogravure equipment has been used to engrave cylinders having large cell areas for receiving viscous liquids such as glue, such rotogravure engravers of the past were not suitable for engraving intaglio patterns having one or more fine or thin lines on cylinders used in printing. This is partly because the rotogravure engravers typically engraved cups or cells having a rhomboidal shape similar to the engraved cells shown in FIG. 6A. Also, cells were not generally rectangular or square in cross section, but rather had a more concave shape as shown in FIG. 6B. Thus, sharp contrasts and print densities were difficult to achieve. The engraving heads which engraved those cells typically had a rise time which is the time required to engrave from a white depth to a full black depth. The rise time of prior art engravers was on the order of about 300 microseconds.
The shape of the cells was dictated in part by the shape of a stylus used to engrave the cells and the speed at which the cells were being engraved. A typical stylus of a gravure engraver of the past is shown in FIG. 5. The typical rise time for an engraving head using this stylus was on the order of about 300-400 microseconds. Because of the very slow rise time of the engravers of the past, it was very difficult to engrave relatively thin, deep horizontal lines (i.e., lines which were generally parallel to the axis of the cylinder) or any thin intaglio lines which were not vertical (i.e., generally perpendicular to the axis of the cylinder).
Unfortunately, this rise time was not adequate to perform intaglio engraving because the height of an engraved column or portion of a character (i.e., the portion of the character that was generally parallel to the rotational axis of the cylinder that can be engraved) is limited by the response time of the engraver and associated time T (FIG. 6B) it takes for the engraver to go from a white area X to a full depth black area Y. The height of such a line and the time T is, in turn, inversely related to the speed of the engraving head and the rotational speed of the cylinder. To increase the response time, the rotational engraving speed of the cylinder had to be slowed down. This also meant that the overall time it took to engrave was increased.
Also, intaglio patterns which had large areas engraved on the cylinder did not print well because the flow of ink into and out of the engraved areas was not controlled well.
Thus, the gravure engravers of the past were limited in design and ability when engraving fine line intaglio patterns.
What is needed, therefore, is a method and apparatus capable of engraving intaglio patterns at acceptable engraving speeds.